There are numerous types of apparatus for measuring the level of a flammable liquid, for example there are apparatuses based on detecting the displacement of a float (direct reading apparatuses, apparatuses having a float suspended on a thread which is wound on a servo-controlled drum, . . . ), or on measuring variation in an electrical property (capacitance probes, resistance probes).
Because of the flammable nature of the liquid whose level is to be measured, all apparatuses which make use either directly or indirectly of electrical components must satisfy various very strict regulations requiring the apparatus to be intrinsically safe by design, or at least requiring the electrical components to be enclosed in flame-proof enclosures.
Apparatuses that make use of an Archimedes displacement plunger generally use an elastic suspension for the plunger (a system including a return spring or a torsion tube), thereby setting up a hysteresis effect and giving rise to errors in measurement which are made worse by analog electronic or pneumatic conversion devices.
Other known apparatuses, likewise using displacement plungers, constitute lever balances having the plunger connected to a system of levers which acts on gearwheels to move a pointer over a graduated scale in a read-out device. In particular, French patent No. 836,857 describes a level meter for brewing vats having a plunger capable of rising and falling depending on the depth of the liquid: the meter includes a housing receiving a system of five articulated levers which connect the plunger to a gearwheel engaging a smaller gearwheel whose shaft carries the pointer, said shaft being further connected on one side to a counterweight and on the other side to a braking dashpot.
Meters of this type are not suitable for measuring the level of a flammable liquid in a tank (measuring accuracy too low if the level varies quickly, and no genuine intrinsic safety), nor are they suitable for measuring the level of very low temperature cryogenic fluids which give rise to particularly severe operating conditions.
Further, such meters which are of traditional design, are unsuitable for use with modern measuring means that do not require a pointer to move over a graduated dial; for example they are unsuitable for use with means using pairs of coded disks as illustrated in U.S. Pat. No. 4,342,996. It should be observed that the means described in said U.S. patent make use of a large number of electrical components such that using them for measuring the level of a flammable liquid would require a highly inconvenient level of intrinsic safety to be provided.
The state of the art is further illustrated by Swiss patent No. 426,310 and U.S. Pat. No. 3,558,861 which describe other known apparatuses serving respectively to determine the Ochsle degree of wine must (or beer wort), and to observe fluctuations in water level in installations for distributing and/or storing water.
Finally, mention should also be made of measurement techniques that use electro-optical means of the type conventional for laboratory balances, and which include a moving scale having uniformly spaced transparent regions which correspond to a variable speed moving diaphragm (Belgian patent No. 529,035).
In general, in the special case of very low temperature cryogenic fluids, in particular liquid hydrogen, prior apparatuses do not give fully satisfactory results, given the very special conditions of temperature (15.degree. K., i.e. about -258.degree. C.), of pressure (60 bars), and of low density (0.07) applicable to liquid hydrogen.
One of the aims of the invention is to provide apparatus for measuring the level of a liquid, and in particular a flammable liquid, in which:
absolute safety is ensured by virtue of the total absence of any electrical components inside the tank or even in the proximity thereof;
very high measurement accuracy is ensured even if the level varies at high speed (when measuring the level of a propellant component in a tank feeding a rocket engine, it is essential to be able to measure depth to within a millimeter over a range of about 4 meters);
measurement is not disturbed by the fact that the level may vary either quickly or slowly; and
it is possible to ensure that the tank remains completely fluid-tight since no member passes through the tank wall.
Further, it will be seen that another essential advantage of the invention is its ability to deliver digitized data representative of depth directly, said digitizing taking place in situ without using any electrical or electronic components or systems.